In order to increase the data transmission rate of fibre-optic communication systems, multiple closely packed wavelength channels are commonly transferred over the same fibre. Such wavelength division multiplexing (WDM) systems may employ in-line optical amplification, e.g. by use of Erbium-doped fibre amplifiers (EDFAs), which work to amplify the optical data transferred in all the wavelength channels simultaneously.
In some cases, e.g. when transmitting over extended distances, signal degradation builds up to a level where signal regeneration is desirable, i.e. to improve the dynamic range of the signals. However, regeneration of all the channels in a WDM system so far require demultiplexing the signals to split out each individual wavelength channel, performing regeneration either optically or electrically, and re-multiplexing the signals into an output WDM signal. Such a system 1 is schematically shown in FIG. 1, where an incoming WDM signal 2 has degraded after transmission over a fibre span 3, to the point where regeneration is desired. The WDM signal is demultiplexed into individual data paths in a demux unit 4, after which each of the channels are reshaped and possibly retimed in individual regenerators 5, before being multiplexed together into a WDM signal again in a mux unit 6. Finally, the WDM signal is re-amplified in an amplifier, such as an EDFA 7. Re-shaping and re-amplification together being regeneration of the signal (so-called “2R”-regeneration, and “3R”-regeneration if retiming is also performed).
Regeneration of multiple wavelength channels have been demonstrated in the electrical domain, i.e. by first detecting the demultiplexed optical signals to generate parallel electrical signals, performing the regeneration and subsequently re-transmitting the signals as optical data. However, electrical systems are limited as to the data rates obtainable.
US2006171716A1 discloses an optical regeneration system for WDM signals. The system disclosed comprises deinterleaving the incoming WDM signal into four channel sets, to increase the channel spacing within each set. To limit the nonlinear interaction between channels in a set, a concatenation of sections of highly nonlinear fibre and dispersion-compensating periodic-group-delay devices (PGDD) are used. The PGDD are designed to provide a carefully selected group delay for the different spectral components in each channel of a set. Thus, the PGDD limits the operation of the regeneration system to a fixed number of channels, at fixed wavelengths. Furthermore, the disclosed regenerator system only works for amplitude modulated signals, i.e. on-off keyed (OOK) signals. As the incoming signals are deinterleaved into four channel sets, four parallel regenerators are needed, thus adding to the system complexity and energy consumption.
US 2001/0021288 A1 discloses a method for waveform shaping of WDM signal light. This method includes the steps of supplying signal light to a first waveform shaper to obtain intermediate signal light, dividing the intermediate signal light into first and second signal lights, supplying the first signal light to a clock recovery circuit to obtain a clock pulse, and supplying the second signal light and the clock pulse to a second waveform shaper to obtain regenerated signal light synchronous with the clock pulse.
US 2005/0185965 A1 discloses an optical synchronizer, which synchronizes the timing of signal light with a plurality of wavelengths (i.e. a WDM signal), the timing of which is not synchronized (asynchronous) in terms of time. Conversion of the synchronized WDM signal to an optical time domain multiplexing (OTDM) signal via an optical gate is also disclosed
Hence, an improved optical regeneration system for WDM signals would be advantageous, and in particular a more energy efficient regeneration system would be advantageous. Furthermore, a regeneration system suitable for phase-encoded signals would be advantageous.